MnO₂ Nanowire–CeO₂ Nanoparticle Composite Catalysts for the Selective Catalytic Reduction of NO_x with NH₃

MnOx-based catalysts have been applied to the selective catalytic reduction of NOx with ammonia (NH3) owing to their high NOx removal efficiency and catalytic stability. In general, the fabrication of a variety of nanomaterials in a complex structure requires complicated processes, including heat treatment and a series of cleaning steps. In addition, MnO2 which has diverse polymorphs, exhibits different catalytic effects depending on its crystalline structure. Among them, synthesizing the ε-MnO2 phase, which functions as a nanocatalyst, has been the most difficult and has hardly been reported. Here, we report the synthesis of heterostructured composite nanocatalysts consisting of ε-MnO2 nanowires (NWs) and CeO2 nanoparticles (NPs) by applying pulsed currents sequentially. This method drastically simplifies the overall process compared to the conventional techniques. Through X-ray diffraction and transmission electron microscopy, it was confirmed that 2–3 nm of CeO2 NPs were formed on the surfaces of the ε-MnO2 NWs. The de-NOx efficiency of the nanocatalysts was analyzed in terms of content variation, specific surface area, and the elemental chemical state of the surface. A ceramic filter containing the nanocatalysts shows a high catalytic activity over the broad operating temperature range 100–400 °C. In the low-temperature region, ε-MnO2 plays a major role in determining the catalytic property, which is consistent with the Brunauer−Emmett−Teller (BET), H2 temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) results. On the other hand, in the high-temperature region, the efficiency increases gradually as the content of CeO2 increases. The H2 TPR, NH3-temperature-programmed desorption, and XPS patterns reveal why the composite exhibits such superior characteristics in the temperature range mentioned above.